GLOBAL EOCENE-OLIGOCENE BOUNDARY UNCONFORMITY IN CLASTIC SEDIMENTARY BASINS

Global sedimentary hiatuses are well-documented in ancient pelagic sediment (based primarily on results of the IODP, DSDP, and ODP programs), and include Paleocene, Eocene-Oligocene boundary, and Miocene hiatuses. Less clear is the extent of these hiatuses into continental margin settings. We test the hypothesis that global hiatuses evident in pelagic sections are also manifested in siliciclastic basins of continental margins globally. We chose to focus on the Eocene-Oligocene boundary and surveyed literature on siliciclastic basins to produce a global inventory of unconformities of this age. The Eocene-Oligocene boundary is a period of global climatic transition characterized by cooling and attendant sea-level changes. Except for South America, which we have yet to investigate, we find evidence for an Eocene-Oligocene unconformity in sedimentary basins along the continental margins of every continent. An Eocene-Oligocene unconformity is present at 81 of 106 sites examined. Diverse oceanographic and sedimentary processes may be responsible for the development of widespread unconformities. Hiatuses in pelagic sections have been attributed to nondeposition, increased corrosiveness of bottom waters and attendant carbonate dissolution, and erosion associated with the intensification of ocean circulation. On clastic continental margins, additional mechanisms must be considered. For example, along the west African margin, sea-level lowering and attendant subaerial shelf exposure have been invoked to explain widespread Eocene-Oligocene erosional unconformities. However, we find that submarine erosion surfaces in deep-water settings are also common in clastic sections at the Eocene-Oligocene boundary. We speculate that one likely mechanism results from the lowered global temperatures and expansion of polar ice at this time, which accelerated ocean thermohaline currents and likely enhanced the potential for submarine erosion.